1. Field of the Invention
This invention relates to improved poly(vinyl chloride) compositions, especially those suitable for injection molding, and to a process for preparing such compositions.
2. Description of the Prior Art
It is known that processing of rigid poly(vinyl chloride), that is, polymers containing at least 80 weight percent vinyl chloride units and containing little or no plasticizer, is extremely difficult to accomplish without the use of polymeric processing aids. Such processing aids when used at levels of from about 0.1 to about 10 parts per 100 parts of poly(vinyl chloride) (PVC), more usually from about 0.5 to about 10 phr (parts per 100 parts of PVC), cause the PVC to flux under heat into a thermoplastic leathery state on mill roll or other similar mixer. The processing aids further allow the PVC to be processed in an extruder into a molten thermoplastic state without the need for high shear forces and temperatures. They further impart to the processed product smoother, uniform surfaces.
A thorough description of processing aids for PVC may be found in "Thermoplastic Polymer Additives, Theory and Practice," Edited by John T. Lutz, Jr., Marcel Deckker, Inc., New York and Basel, 1988 (chapter by R. P. Petrich and John T. Lutz, Jr.) and "History of Polymer Composites," Edited by R. B. Seymour and R. D. Deanin, VNU Science Press BV, Netherlands, 1987 (chapter by D. L. Dunkelberger).
Particularly useful as processing aids have been high molecular weight polymers which are predominately composed of units derived from methyl methacrylate, which have been commercially available for about 30 years. These additives are especially useful in that they do not distract from the important physical properties expected from the final processed PVC object, such as heat distortion temperature, tensile strength, and impact-resistance properties.
One deficiency that they do have is that the high molecular weight polymers, even at the low levels used, either maintain or increase the melt viscosity of the resultant blend. This is especially undesirable in injection molding applications, where it is desired to have a melt of low viscosity for ease in mold-filling. Lowering of the molecular weight of the processing aid will decrease the melt viscosity of the blend, but sometimes at the sacrifice of the heat distortion temperature of the final precessed object.
Similar effects (improved flow at the cost of lowered heat distortion temperature) are found with other non-polymeric additives, such as plasticizers, or when a lower molecular weight PVC or a PVC copolymer is used.
The effect sought herein, wherein a high molecular weight polymer blend on heating to process exhibits anomalously low melt viscosity, yet reverts to the performance expected from the high molecular weight polymer upon cooling, has been accomplished in certain limited systems by use of a "solid solvent" which is compatible with the polymer matrix on cooling, yet melts and plasticizes the matrix polymer on heating. Such "solid solvents" are rare, unknown for PVC, and still offer deficiencies in end use application, such as extractability with solvents, contribution to taste and odor of material packaged therein, or lack of permanence.
A second approach is to use as the additive a liquid crystalline polymer which forms a mesophase on processing. Such specific copolymers are expensive for the purpose herein, which is to modify the rheological behavior of a high-volume low-cost thermoplastic and low add-on cost.
There thus has been a long-felt need for an additive which will allow PVC to be processed in injection molding to useful objects which maintain the physical properties of a medium- to high-molecular weight PVC, such as heat distortion temperature and toughness, while being capable of being molded under commercially practical and safe conditions of temperature, time, and pressure.